In peel-apart type instant films (which are hereinafter referred as film units for simplicity). two separate diffusion transfer sheets are used in black-and-white or monochromatic photography: a negative or photosensitive sheet and an image receiving or print sheet where a positive image is formed.
For the purpose of providing a brief background that will enhance an understanding of the construction of the film unit in which the present invention is embodied, reference is now had to FIG. 1. The negative and positive sections of the film unit are on separate sheets: a negative or photosensitive sheet, and a positive or image-receiving sheet. In the photosensitive sheet 1, exposure creates a latent image. Connected to the photosensitive sheet 1 are a withdrawing sheet member 2 at its front margin (at the right-hand side of the photosensitive sheet in FIG. 1) and a rear flap member 3 at its rear margin. The withdrawing sheet member and flap member 3 are both made of black carbon containing paper sheet materials which have less surface reflection. The withdrawing sheet member 2 is provided with a rectangular tab 4 detachably connected thereto near its front section. The withdrawing sheet member 2 is further provided on its rear portion with a rupturable pod 5 containing a processing reagent or a liquid or viscous developer reagent therein, as well as a funnel member 2a which is adapted to cooperate with the rear portion of the withdrawal sheet member 2 so as to form a passage for the developer reagent released from the pod 5 for preventing it from escaping from the sides of the film unit.
A mask member 6, which is connected to the withdrawing sheet member 2 at its front end, is formed with an aperture 6a for defining an image receiving area 7a on the image receiving sheet 7 which is attached to the back of the mask member 6 in such a way as to cover the aperture 6a. Along each side of the aperture 6a, the mask member 6 is provided with a predetermined thickness of rail for effecting a uniform distribution of developer reagent. The mask member 6 has at its rear end a flap 6b which is adapted to face the rear flap member 3 at the end of the withdrawal of the film unit between a pair of pressure applying processing rollers of which operation and fabrication is well known to those skilled in the art and need not be explained in detail herein. To each rail member 5 on the flap 6b a spacer member 9 is adhered to provide a trapping space for developer reagent.
All of the members 2, 3 and 9, which are made of paper sheet materials are adhered to the photosensitive and/or image receiving sheets 1 and/or 7 through sealing material.
The film unit thus constructed is, after exposure, withdrawn from an instant camera or a camera back. At this time the two sheets 1 and 7 pass between the pair of juxtaposed pressure applying processing rollers thereby to be superimposed emulsion-to-emulsion and to break the pod so as to release the viscous developer reagent and spread it between the two superimposed sheets 1 and 7 in a thin uniform layer after having passed through the passage formed by the rear portion of the withdrawing member 2 and the funnel member 2a. The developer reagent thus spread between the two superimposed sheets 1 and 7 is maintained uniform in thickness by virtue of the spacer rails 8, so as to avoid unevenness in development over the image area. In a very few minutes, a positive image is formed on the image-receiving sheet by means of diffusion transfer.
As is shown in FIG. 2, the photosensitive sheet 1 generally consists of various materials in layers: a polyethylene (which is hereinafter abbreviated "PE") layer 25, a gray paper layer 26, a PE layer 27 and photosensitive emulsion layer 28. Additional details may be obtained by reference to U.S. Pat. No. 219,159.
The PE layer 25 which contains carbon is 10-48 g/m.sup.2 in weight and serves as a black opaque layer. The paper layer 26 which contains carbon and titanium dioxide and is gray is 55-125 g/m.sup.2 in weight and used as a paper base support. The carbon is added to connect fibers by filling the joints (pin holes). On the other hand, the titanium dioxide is added to compensate for a lack of whiteness, namely reflectance, of the PE layer 27 so as to improve the reflectance of the PE layer 27. The PE layer 27 which contains enough titanium dioxide to effect a masking of the carbon contained in the paper layer 26 and is white is 10-48 g/m.sup.2 in weight.
In recent monochromatic instant film, fast films are required such as to have speed as or sensitivities rated 3000 or more in ISO (International Standardization Organization) speed number code. In an attempt at realizing such a fast instant film it will be effective to use exposing light sufficiently as well as to improve the emulsion layer. For best performance of exposing light, it is advantageous to return light passed through the emulsion layer 28 to the emulsion layer 28.
Exposing light can be reflected by the PE layer 27 with an increased reflectance and so as to be returned to the emulsion layer. Reflectance can be increased by adding a large amount of titanium dioxide in the PE layer 27. However, as is well known to those in the art, for preventing a PE layer of an instant film from being cracked when withdrawn from a camera or a camera back, the amount of added titanium dioxide is restricted up to approximately 17% in weight desirably to a practical range between 13 and 15% in weight. For this reason, the paper layer is needed to have an increased reflectance by virtue of an increased amount of titanium dioxide so as to reflect exposing light imaging onto the front surface thereof. However, because of a limited amount of additives of the paper layer, more titanium dioxide is increasingly added in the paper layer, lesser carbon can be containing. If the amount of carbon contained in the paper layer is in fact decreased, it becomes difficult to obtain an instant film with a sufficient light shielding effect.